As a general example of a prior art device generating plasma by charging a plate-like electrode with a radio frequency power, a construction of a plasma CVD device generating plasma to form a silicon film or the like on a target substrate will be described with reference to FIG. 4. Also, a radio frequency power supply structure supplying this electrode with the radio frequency power from an RF cable will be described with reference to FIGS. 5 and 6. FIG. 4 is a schematic constructional view showing one example of a prior art plasma CVD device. FIG. 5 comprises FIG. 5(a) and FIG. 5(b), wherein FIG. 5(a) is a perspective view of a ladder electrode and an RF cable both shown in FIG. 4 and FIG. 5(b) is a side view as seen in the direction of arrow C of FIG. 5(a). FIG. 6 is an enlarged cross sectional view of portion D of FIG. 5(b).
In the plasma CVD device, designated by reference numeral 100 in FIG. 4, plasma is generated by the electrode being charged with the radio frequency power, so that a film of amorphous silicon, microcrystalline silicon, polycrystalline silicon, silicon nitride, etc. to be used for a solar cell, thin-film transistor, etc. is formed on a substrate 5. Recently, there is a large demand for the substrate 5 of a large size and, in order to uniformly apply a silicon film onto the large size substrate, the electrode for the radio frequency power is also constructed in a plate shape and in a large size. As an example thereof, there is provided a ladder electrode 3, as shown in FIGS. 4 and 5. That is, two lateral electrodes 3a forming two mutually opposed end peripheral portions of the electrode of the plate shape are connected to each other via a plurality of longitudinal electrodes 3b so that the ladder electrode 3 formed in a longitudinal grid plate shape is provided.
Herein, while the description of both of the prior art and embodiments of the present invention will be made based on the example using the ladder electrode 3 as a plate-like electrode for the radio frequency power, the present invention is not limited to the case using the ladder electrode 3 but is also applicable to the radio frequency power supply structure using a general radio frequency electrode formed in a plate shape.
In the prior art plasma CVD device 100 exemplified in FIG. 4, the ladder electrode 3 is provided in a vacuum vessel 1. On one side of the ladder electrode 3 in the vacuum vessel 1, an earth electrode 4 is provided and the substrate 5, mentioned above, to which the silicon film is to be applied is arranged on the earth electrode 4. On the other side of the ladder electrode 3, not on the substrate 5 side, a film forming chamber 2 is formed so as to surround the ladder electrode 3. The film forming chamber 2 has its substrate 5 side opened and also has its inner wall surface facing to the ladder electrode 3 fitted with a gas supply portion 6 connected to a gas supply source (not shown). The vacuum vessel 1 comprises an exhaust pipe 16 through which gas is discharged outside. The exhaust pipe 16 is connected to an exhaust device, that is, a vacuum device (not shown).
The lateral electrode 3a comprises a connecting portion 11 to which an RF cable 9 is connected, so that the ladder electrode 3 is supplied with a radio frequency power i from an RF power source (radio frequency power source) 7 via a matching device 8. It is to be noted that the connecting portion 11 is usually provided in the number of plurality in order to evenly supply the ladder electrode 3 of the plate shape with the radio frequency power.
By the radio frequency power supplied to the ladder electrode 3, silane gas (SiH4) supplied from the gas supply portion 6 is changed to plasma in a space 10 between the ladder electrode 3 and the substrate 5 in the vacuum vessel 1 and a film of silicon (Si) is formed on the substrate 5.
Also, if nitrogen trifluoride (NF3) is injected for removing the silicon film deposited on the ladder electrode 3, etc. in the vacuum vessel 1, the radio frequency power supplied to the ladder electrode 3 can be used for changing the nitrogen trifluoride to plasma that can be decomposed as well as for etching the interior of the vacuum vessel 1 by F (fluorine) radical generated by the decomposition. Also, the radio frequency power can otherwise be used for various surface treatments using plasma.
In order to make possible a high-speed film forming or a surface treatment of a high-speed etching or the like, it is necessary to generate a high density plasma and, for this purpose, the electrode is needed to be supplied with the radio frequency power with a high efficiency. However, in the prior art radio frequency power supply structure using the ladder electrode 3, there is a problem as follows.
That is, as shown in FIGS. 5(a) and 5(b), the RF cable 9 is connected to the ladder electrode 3 with a right angle θ relative to a plane formed by the ladder electrode 3 at the connecting portion 11 of the lateral electrode 3a. 
Hence, the power i from the RF cable 9 streams into the lateral electrode 3a turning orthogonally at the connecting portion 11. Also, in the case where the longitudinal electrode 3b directly connects to the connecting portion 11, the power i from the RF cable 9 streams into the longitudinal electrode 3b turning orthogonally at the connecting portion 11.
Such radio frequency power supply structure of the connecting portion 11 in the prior art through which the power i streams into the ladder electrode 3 will be described with reference to FIG. 6. In FIG. 6, numeral 9a designates an RF cable core wire, numeral 9b an RF cable outer shell that functions as earth and numeral 9c an insulator portion.
While the power i passes through the interior of the RF cable 9, a voltage acts between the RF cable outer shell 9b and the RF cable core wire 9a so that the power i is stably transmitted therein. But, as the RF cable outer shell 9b is cut off at the connecting portion 11 of the ladder electrode 3 so that the earth is also cut off there, the voltage after the connecting portion 11 acts relative to an arbitrary earth and an electric line of force e becomes unstable there.
Also, as the RF cable 9 connects to the connecting portion 11 orthogonally to the plane formed by the ladder electrode 3, the voltage acting after the connecting portion 11 becomes asymmetric and thereby a distribution of the electric line of force e also becomes asymmetric. Hence, an impedance at the connecting portion 11 becomes largely changeable.
For this reason, a reflection of the radio frequency power arises at the connecting portion 11 and there is caused a problem that an incidence of the radio frequency power i into the ladder electrode 3 is reduced, thereby worsening the efficiency of the film forming and the surface treatment.
Moreover, as the reflection of the power i at the connecting portion 11 is large, if there is an error in setting the connection at the connecting portion 11, there are caused differences in the reflecting power and thereby the incident power is also largely influenced. Hence, in case where the electrode structure has a plurality of the connecting portions 11, there are caused imbalances of the incident power i between each of the plurality of the connecting portions 11. These imbalances cause a non-uniformity of the power in the plane formed by the ladder electrode 3 to worsen the uniformity of plasma thereby generated. Thus, in the prior art, there is a problem to often deteriorate the quality of products on which the film is formed or the surface treatment is carried out.